A liquid crystal display device controls the light transmittance of liquid crystal by use of an electric field, thereby displaying a picture. The liquid crystal display device is generally classified into a vertical electric field applying type and a horizontal electric field applying type in accordance with the direction of the electric field which drives the liquid crystal.
The vertical electric field applying type liquid crystal display device drives a liquid crystal of a VA (vertical alignment) mode by a vertical electric field which is formed between a pixel electrode and a common electrode which are disposed to face each other in upper and lower substrates. The vertical electric field applying type liquid crystal display device has an advantage in that an aperture ratio thereof is high, but on the contrary has a disadvantage in that a viewing angle thereof is narrow to be about 90°.
The horizontal electric field applying type liquid crystal display device drives a liquid crystal of an IPS (in plane switch) mode by a horizontal electric field between a pixel electrode and a common electrode which are disposed to be parallel in a lower substrate. The horizontal electric field applying type liquid crystal display device has an advantage in that a viewing angle thereof is wide to be about 160°. Hereinafter, the horizontal electric field applying type liquid crystal display device will be described in detail.
The horizontal electric field applying type liquid crystal display device includes a thin film transistor substrate (lower substrate) and a color filter substrate (upper substrate) which are bonded to face each other; a spacer for fixedly keeping a cell gap between the two substrates; and a liquid crystal filled in the cell gap.
The thin film transistor substrate includes a plurality of TFT's and a plurality of signal lines for forming a horizontal electric field by the unit of a pixel; and an alignment film spread thereover for aligning the liquid crystal. The color filter substrate includes a color filter for realizing color; a black matrix for preventing light leakage; and an alignment film spread thereover for aligning the liquid crystal.
FIG. 1 is a cross sectional view illustrating a thin film transistor substrate and a color filter substrate of a horizontal electric field applying type liquid crystal display device of the related art.
Referring to FIG. 1, a thin film transistor substrate 70 includes gate lines and data lines 4 which are formed to cross each other on a lower substrate 33; thin film transistors formed at each crossing part thereof; and common electrodes 18 and pixel electrodes 14 for forming a horizontal electric field in a pixel area provided by the crossing structure.
The thin film transistor charges the pixel electrode 14 with a pixel signal of the data line 4 and keeps the charged pixel signal in the pixel electrode 14 in response to a gate signal of the gate line. To this end, the thin film transistor includes a gate electrode connected to the gate line, a source electrode connected to the data line 4 and a drain electrode which forms a channel therebetween together with the source electrode and which is connected to the pixel electrode.
The pixel electrode 14 is connected to the drain electrode of the thin film transistor to be formed in the pixel area.
The common electrode 18 is connected to a common line to be formed in the pixel area.
The gate line and the data line 4 receive a signal from a gate driver and a data driver through a gate pad and a data pad.
The common line receives a common voltage from a common voltage source through a common pad.
Accordingly, a horizontal electric field is formed between the pixel electrode 14 to which the pixel signal is supplied through the thin film transistor and the common electrode 18 to which the common voltage is supplied through the common line.
The color filter substrate 80 includes a black matrix 36 which prevents light leakage and absorbs an external light to increase contrast; a color filter 38 for realizing color; and an overcoat layer 40 for leveling the color filter 38.
Liquid crystal molecules 41 horizontally arranged between an alignment film 39 of the thin film transistor substrate 70 and an alignment film 42 of the color filter substrate 80 are rotated by a dielectric anisotropy caused by the horizontal electric field. Accordingly, the transmittance of the light passing through the pixel area is changed in accordance with the degree of rotation of the liquid crystal molecules 41, thereby realizing a gray level.
A lower polarizer 31 located at the rear surface of the thin film transistor substrate 70 and an upper polarizer 32 located at the rear surface of the color filter substrate 80 are disposed for light transmission axes thereof to be at right angles to each other. In other words, the light passing through the liquid crystal molecules 41 passes through the upper polarizer 32 if the linearly-polarized light is changed by the liquid crystal molecules 41, but the light cannot pass through the upper polarizer 32 if the linearly-polarized light is not changed by the liquid crystal molecules 41.
On the other hand, if black is realized by such a horizontal electric field applying type liquid crystal display device, the light linearly polarized by the lower polarizer 31 is not sufficiently absorbed to the upper polarizer 32, thus there appears a light leakage phenomenon that the amount and color characteristic of the light becomes different in case of observing a location which escapes from the front of the liquid crystal display device, i.e., a side surface thereof, in comparison with observing the front thereof. Especially, as shown in FIG. 2, if the viewing angle is ±70°, the light transmittance is high, thus the light leakage is generated in the greatest scale. This is because the upper and lower polarizers 32, 31 have a structure where a passivation layer having a light absorption axis is deposited with a polarizer having a light transmission axis therebetween, and the passivation layer has a one axis characteristic of having a fixed delay value, thus the polarization direction of the upper polarizer is changed.
The light leakage phenomenon is also generated in the vertical electric field applying type liquid crystal display device that uses the polarizer as well as the horizontal electric field applying type liquid crystal display device shown in FIG. 1.
In order to reduce the light leakage phenomenon, a compensation film such as an A-plate, a positive C-plate, a biaxial film, etc, as shown in FIG. 3, is adhered to the rear surface of the upper and lower substrates 34, 33 together with the upper and lower polarizers 31, 32. The compensation film adhered to the upper substrate 34 uses a structure where a first upper compensation film 51a and a second upper compensation film 51b are deposited, and the compensation film adhered to the lower substrate 33 uses a single layer structure of the lower compensation film 53. At this moment, a separated type film composed of the A-plate and the positive C-plate is used for the upper compensation film 51a, 51b. The light axis of the A-plate has the same horizontal axis as the light transmission axis of the upper polarizer 31, and the light axis of the positive C-plate has the axis which is vertically formed. As shown in FIG. 2, the light leakage phenomenon is greatly reduced by the compensation films.
However, because such a compensation film is additionally required, there is generated a problem in that the cost is increased in comparison with the liquid crystal display device of the related art.